1. Field of the Invention
The invention relates to an apparatus for information communication systems having a communication medium and, more particularly, this invention relates to an apparatus for information communication systems which allows avoidance of collisions of information signals within a communication medium.
2. Description of the Prior Art
An example of an anti-collision device for an information communication system (sometimes referred to as an inter-system-link) is shown in U.S. Pat. No. 4,063,220 issued to Metcalfe et al on Dec. 13, 1977 and entitled "Multi Point Data Communication System With Collision Detection" and U.S. Pat. No. 4,099,024 issued to Boggs et al on July 4, 1978 and entitled "Communications Network Repeater". The system shown in the Metcalfe and Boggs patents utilizes a plurality of repeaters to aid in reducing the possibility of undetected collisions by reducing attenuation. The Metcalfe and Boggs patents show collision detection by utilizing a device which determines when a collision has actually occurred by measuring phase shift in the information signal received.
A further discussion of detecting and avoiding collisions within information communication systems is shown in an article by Kleinrock entitled "Performance of Distributed Multi-Access Computer Communication Systems" published in the 1977 IFIP Congress Proceedings at page 587. The methods described include the one discussed above in connection with the Metcalfe and Boggs patent. Another method described is to utilize a time slot during which each device connected to the information communication system is authorized to transmit. Another method is to have a central controller send a request to each device connected to the information communication system to determine if it has information which it desires to transmit. A further method is to divide the information communication system over the frequency spectrum and provide each device on the information communication system with a different bandwidth for transmission purposes. Another method described in the article is to have each receiver connected to each device on the information communication system determine if any carrier is detected on the link. The Kleinrock article generally discusses the various problems with the eight schemes discussed therein (some of the schemes are combinations of the various other schemes), which Kleinrock refers to as the unavoidable price.
The basic problem is that one transmitter may begin to transmit without realizing that another transmitter is transmitting because the communication medium attenuates the information signal transmitted and causes propagation delays for the information signals transmitted therethrough.
None of the prior constructions or methods of collision avoidance utilize a device which provides a predetermined voltage level DC to the communication medium (coaxial cable) of the information communication system, examines the DC voltage level of the communication medium, and compares that voltage level with the predetermined D.C. voltage level provided to the communication medium.